1. Field of the Invention
The present invention relates to amplifiers, and more particularly to controlled-gain audio or video bandwidth amplifiers with a precise exponential relation between gain and control voltage.
2. Description of Related Art
Voltage-controlled variable gain amplifiers are used in Automatic Gain Control (AGC) circuits and Sensitivity-Time-Control (STC) circuits in sampling-type radar rangefinders and in ultrasonic rangefinders. AGC circuits do not require high gain-control accuracy since amplifier gain is regulated by a feedback loop responsive to signal amplitude, where gain-control inaccuracies are substantially reduced by the control loop gain. Consequently, most AGC circuits are able to use low-accuracy variable transconductance elements such as vacuum tubes, PN junctions, or FETs as the voltage-controlled gain element.
STC circuits increase receiver gain as a mathematical function of the time after a pulse is transmitted (or equivalently, the range to the target). Usually, the gain relation varies with R.sup.2, where R is the radar or ultrasonic range to a target. This is a natural relation due to wave expansion as a function of range. However, if a target is very large, such as a pool of water at the bottom of a large tank, the STC gain relation varies with R.sup.1. Another situation occurs when propagating through a lossy medium such as soil for ground penetrating radar (GPR) or through air for ultrasonic rangefinders. In this case the STC gain relation might vary with R.sub.x, where x is a non-integer exponent generally &gt;2.0 that is materials dependent. Thus there is a need for user-adjustable continuously-variable gain relation STC exponent control.
STC circuits, like AGC circuits, provide a large gain-control range in most receiver applications, perhaps 60 dB or greater. In AGC circuits, the output is regulated, eliminating the possibility of signal saturation. STC circuits, however, must have an accurate control relation since a large amount of gain is controlled by a range-dependent voltage on an open-loop basis. When variable transconductance elements are used in STC circuits, a high reliance must be placed on matched elements, circuit symmetries, and other precision analog techniques to achieve high accuracy. Precision analog elements are expensive and there is a limited selection of suitable devices.